Synthesis gas (syngas) containing hydrogen and carbon oxides is an important feedstock for the production of a wide range of chemical products. Syngas mixtures having controlled ratios of hydrogen and carbon monoxide are catalytically reacted to produce liquid hydrocarbons and oxygenated organic compounds including methanol, acetic acid, dimethylether, oxoalcohols and isocyanates. The syngas product can be further processed and separated to yield high purity hydrogen and carbon monoxide. The cost of generating the syngas is frequently the largest part of the total cost of preparing these products.
Two major reaction routes are commonly used by industry for syngas production, namely steam reforming of light hydrocarbons, primarily natural gas, naphtha and refinery offgases, and the partial oxidation of carbon-containing feed stocks ranging from natural gas to high molecular weight liquid or solid carbonaceous materials. Autothermal reforming is an alternate process which uses a light hydrocarbon feed which combines features of partial oxidation and steam reforming reactions in a single reactor. A concise review of such processes is described in U.S. Pat. No. 6,077,323. Such processes typically require oxygen in purities of greater than 95 vol %, which is available from cryogenic air separation in large tonnage volumes or pressure swing absorption for smaller volumes.
Alternative processes have been developed for syngas production wherein oxygen necessary to conduct the partial oxidation reaction is provided in situ by the separation of air at high temperatures using solid-state membranes which conduct oxygen ions and electrons under operating conditions. Solid-state membranes which conduct oxygen ions and electrons are also known as mixed conducting membranes. Such mixed conducting membranes can be used in combination with appropriate catalysts to produce syngas in a membrane reactor eliminating the need for a separate oxygen production step. A membrane reactor typically has one or more reaction zones, wherein each reaction zone comprises a mixed conducting membrane which separates each reaction zone into an oxidant side and a reactant side.
Multicomponent metallic oxides are represented in the art by formulae which present one or more xe2x80x9cA-sitexe2x80x9d metals and one or more xe2x80x9cB-sitexe2x80x9d metals. By way of example, U.S. Pat. No. 5,306,411 discloses certain multicomponent metallic oxides having the perovskite structure represented by the formula AsAxe2x80x2tBuBxe2x80x2vBxe2x80x3wOx, wherein A represents a lanthanide, Y or a mixture thereof; Axe2x80x2 represents an alkaline earth metal or mixture thereof; B represents Fe; Bxe2x80x2 represents Cr, Ti or a mixture thereof; and Bxe2x80x3 represents Mn, Co, V, Ni, Cu or a mixture thereof, and s, t, u, v, w and x each represent a number such that s/t equals from about 0.01 to about 100; u equals from about 0.01 to about 1; v equals from about 0.01 to 1; w equals from 0 to about 1; x equals a number that satisfies the valences of A, Axe2x80x2, B, Bxe2x80x2 and Bxe2x80x3 in the formula; provided that 0.9 less than (s+t)/(u+v+w) less than 1.1. In a preferred embodiment Axe2x80x2 is calcium or strontium and Bxe2x80x3 represents Mn or Co or a mixture thereof. These multicomponent metallic oxides require chromium or titanium as a B-site element.
Multicomponent metallic oxides depicted by formulae presenting A-site metals and B-site metals may be stoichiometric compositions, A-site rich compositions or B-site rich compositions. Stoichiometric compositions are defined as materials wherein the sum of the A-site metal stoichiometric coefficients equals the sum of the B-site metal stoichiometric coefficients. A-site rich compositions are defined as materials wherein the sum of the A-site metal stoichiometric coefficients is greater than the sum of the B-site metal stoichiometric coefficients. B-site rich compositions are defined as materials wherein the sum of the B-site metal stoichiometric coefficients is greater than the sum of the A-site metal stoichiometric coefficients.
U.S. Pat No. 6,033,632 discloses a solid-state membrane for use in a catalytic membrane reactor which utilizes a membrane fabricated from a multicomponent metallic oxide having the stoichiometry A2xe2x88x92xAxe2x80x2xB2xe2x88x92yBxe2x80x2yO5+z, wherein A is an alkaline earth metal ion or mixture of alkaline earth metal ions; Axe2x80x2 is a metal ion or mixture of metal ions where the metal is selected from the group consisting of metals of the lanthanide series and yttrium; B is a metal ion or mixture of metal ions, wherein the metal is selected from the group consisting of 3d transition metals and the group 13 metals; Bxe2x80x2 is a metal ion or mixture of metal ions where the metal is selected from the group of the 3d transition metals, the group 13 metals, the lanthanides and yttrium; x and y are independently of each other numbers equal to or greater than zero and less than 2; and z is a number that renders the compound charge neutral. In a preferred embodiment the 3d transition metal is Fe and the group 13 metal is Ga, whereas Axe2x80x2 preferably is La and A is Sr and Ba.
U.S. Pat. Nos. 5,356,728 and 5,580,497 disclose cross-flow electrochemical reactor cells formed from multicomponent metallic oxides which demonstrate electron conductivity and oxygen ion conductivity at elevated temperatures. According to both documents, suitable multicomponent metallic oxides are represented by (Sr1xe2x88x92yMy)xcex1(Fe1xe2x88x92xCox)xcex1+xcex2)xcex4, wherein M is a metal selected from the group consisting of elements having atomic number in a range from 56 to 71, calcium and yttrium, x is a number in a range from about 0.01 to about 0.95, y is a number in a range from about 0.01 to about 0.95, xcex1 is a number in a range from about 1 to about 4, xcex2 is a number in a range upward from 0 to about 20, such that 1.1 less than (xcex1+xcex2)/xcex1xe2x89xa66, and xcex4 is a number which renders the compound charge neutral.
U.S. Pat. No. 6,056,807 teaches a fluid separation device capable of separating oxygen from an oxygen-containing gaseous mixture which utilizes at least one solid-state membrane comprising a dense mixed conducting multicomponent metallic oxide layer formed from a composition of matter represented by the formula:
LnxAxe2x80x2xxe2x80x3Axe2x80x2xxe2x80x3ByBxe2x80x2yxe2x80x2O3xe2x88x92z,
wherein Ln is an element selected from the f block lanthanides, Axe2x80x2 is selected from group 2, Axe2x80x3 is selected from groups 1, 2 and 3 and the f block lanthanides and B and Bxe2x80x2 are independently selected from the d block transition metals, excluding titanium and chromium, wherein 0xe2x89xa6x less than 1, 0 less than xxe2x80x2xe2x89xa61, 0xe2x89xa6xxe2x80x3 less than 1, 0 less than y less than 1.1, 0xe2x89xa6yxe2x80x2 less than 1.1., x+xxe2x80x2+xxe2x80x3=1.0, 1.1 greater than y+yxe2x80x2 greater than 1.0 and z is a number which renders the compound charge neutral. This reference discloses B-site rich non-stoichiometric compositions because the sum of the x indices is 1.0 and the sum of the y indices is greater than 1.0.
U.S. Pat. No. 5,712,220 presents a class of multicomponent metallic oxides which are well suited toward use in fabricating components used in solid-state oxygen separation devices. While the reference relates primarily to B-site rich compositions, the reference discloses A-site rich non-stoichiometric compositions represented by the formula LnxAxe2x80x2xxe2x80x2Axe2x80x3xxe2x80x2ByByxe2x80x2Bxe2x80x3yxe2x80x3O3xe2x88x92z wherein Ln is an element selected from the f block lanthanides, Axe2x80x2 is selected from Group 2, Axe2x80x3 is selected from Groups 1, 2 and 3 and the f block lanthanides and B, Bxe2x80x2 and Bxe2x80x3 are independently selected from the d block transition metals, excluding titanium and chromium, wherein 0xe2x89xa6x less than 1, 0 less than xxe2x80x2 less than 1, 0xe2x89xa6xxe2x80x3 less than 1, 0 less than y less than 1.1, 0 less than yxe2x80x2 less than 1.1, 0 less than yxe2x80x3 less than 1.1, x+xxe2x80x2+xxe2x80x3=1.0 0.9 less than y+yxe2x80x3 less than 1.0 and z is a number which renders the compound charge neutral wherein such elements are represented according to the Periodic Table of the Elements adopted by IUPAC.
A solid-state membrane employed in a process for making syngas is exposed to severe reaction conditions such as temperatures above 600xc2x0 C., a large pressure difference across the solid-state membrane, a highly oxidizing environment on one surface, and a water, hydrogen, methane, carbon monoxide and carbon dioxide containing reactant gas stream on the other surface. Therefore, the solid-state membrane must have sufficiently high oxygen flux, a sufficient chemical stability in the syngas and air environments, a sufficiently low creep rate under the applied mechanical load, a sufficient resistance to demixing of the metal cations and a sufficiently low chemical expansion under the membrane operating conditions.
Numerous compositions known in the art for fabricating solid-state membranes do not adequately meet all the above criteria. Although some compositions are known to meet the oxygen flux criteria, for example, these compositions typically may not meet one or more other criteria as listed above. These criteria are nevertheless highly sought after for development of an economically viable technology based on solid-state membranes.
Researchers continue to search for suitable solid-state membranes that will economically and reliably produce syngas through the oxidation of methane and partially reformed methane feed stocks. More in detail, researchers are searching for mixed conducting multicomponent metallic oxides suitable for use in fabricating the dense layer of a solid-state membrane which meet the above criteria.
Applicants have discovered a new class of A-site rich non-stoichiometric multicomponent metallic oxides which are particularly suited toward use in solid-state membranes suitable for use in processes for producing synthesis gas (syngas). These compositions of matter overcome problems associated with many prior art compositions of matter by providing a favorable balance of oxygen permeance, resistance to degradation, favorable sintering properties and coefficients of thermal expansion which are compatible with other materials used to fabricate solid-state membranes.
The compositions of matter according to the invention are represented by the formula:
(LnxCa1xe2x88x92x)yFeO3xe2x88x92xcex4
wherein
Ln is La or a mixture of lanthanides comprising La, and wherein
1.0 greater than x greater than 0.5
1.1xe2x89xa7y greater than 1.0 and
xcex4 is a number which renders the composition of matter charge neutral.
In a preferred embodiment, 0.98 greater than x greater than 0.75 and 1.05 xe2x89xa7yxe2x89xa71.01.
The invention also presents solid-state membranes which comprise a first side and a second side wherein the solid-state membrane comprises a dense layer formed from a composition of matter represented by the formula:
(LnxCa1xe2x88x92x)yFeO3xe2x88x92xcex4
wherein
Ln is La or a mixture of lanthanides comprising La, and wherein
1.0 greater than x greater than 0.5
1.1xe2x89xa7y greater than 1.0 and
xcex4 is a number which renders the composition of matter charge neutral.
In a preferred embodiment, the dense layer of the solid-state membrane is formed from a composition of matter according to the formula wherein 0.98 greater than x greater than 0.75 and 1.05xe2x89xa7yxe2x89xa71.01.
The solid state membranes may further comprise any number of additional layers to enhance performance and durability. Such additional layers may include a porous mixed conducting multicomponent metallic oxide layer contiguous to the dense layer. The solid-state membranes which comprise a dense layer and any number of additional layers may be fabricated into a variety of shapes including flat plates or tubes. These solid-state membranes possess two exterior sides, referred to as the first side and the second side.
For example, one can envision the two sides of a coin in the case of a flat plate or the interior and exterior surfaces of a tube. Such solid-state membranes may also include a catalyst on the first side, a catalyst on the second side or on the first side and the second side meaning the surfaces of the membrane which will be in contact with the oxygen-containing feed gas and the methane-containing reactant gas during operating of the syngas process of this invention.
By way of example, the first side may be referred to as the reactant side and the second side may be referred to as the oxidant side. Suitable catalysts to be deposited onto the reactant side of the solid-state membrane are conventional reforming catalysts or partial oxidation catalysts such as a metal or an oxide of a metal selected from Groups 5, 6, 7, 8, 9,10, 11 of the Periodic Table of the Elements according to the International Union of Pure and Applied Chemistry. Preferred metals or oxides of metals are selected from the group consisting of nickel, cobalt, iron, platinum, palladium, and silver.
By way of example, the second side of the solid-state membrane may be referred to as the oxidant side. Suitable catalysts to be deposited onto the oxidant side are conventional oxygen reduction catalysts such as a metal or an oxide of a metal selected from the Groups 2, 5, 6, 7, 8, 9, 10, 11 and 15 and the F block lanthanides of the Periodic Table of the Elements according to the International Union of Pure and Applied Chemistry. Preferred metals or oxides of metals are selected from the groups consisting of platinum, palladium, ruthenium, gold, silver, bismuth, barium, vanadium, molybdenum, cerium, praseodymium, cobalt, rhodium and manganese. Optionally, the catalyst may be any multicomponent metallic oxide which catalyzes the desired reaction.
The invention also presents a process for producing a synthesis gas product comprising hydrogen and carbon monoxide comprising the steps of:
a) providing a reaction zone having an oxidant side and a reactant side which are separated by a solid-state membrane comprising a dense layer formed from a composition of matter represented by the formula:
(LnxCa1xe2x88x92x)yFeO3xe2x88x92xcex4
xe2x80x83wherein
Ln is La or a mixture of lanthanides comprising La, and wherein
1.0 greater than x greater than 0.5
1.1xe2x89xa7y greater than 1.0 and
xcex4 is a number which renders the composition of matter charge neutral,
b) heating an oxygen-containing feed gas and introducing the resulting heated oxygen-containing feed gas in the oxidant side of the reaction zone at an oxidant feed temperature and an oxidant gas feed pressure;
c) heating a methane-containing reactant gas and introducing the resulting heated methane-containing reactant gas in the reactant side of the reaction zone at a reactant gas feed temperature and a reactant gas feed pressure;
d) permeating oxygen from the oxidant side of the reaction zone through the solid-state membrane to the reactant side of the reaction zone and reacting the oxygen with the methane-containing reactant gas to form the synthesis gas product;
e) withdrawing the synthesis gas product from the reactant side of the reaction zone at a product gas outlet temperature; and
f) withdrawing an oxygen depleted gas stream from the oxidant side of the reaction zone at a product gas outlet temperature.
The oxygen containing feed gas in step (b) is preferably heated by direct combustion with a fuel in a direct-fired combustor to produce a hot, pressurized combustion product to provide the heated oxygen-containing feed gas.
Optionally, the process further comprises the steps of:
g) introducing a heated gaseous stream comprising steam and one or more hydrocarbons into a catalytic reforming reaction zone comprising at least one catalyst which promotes steam reforming of hydrocarbons to form a partially reformed intermediate gas comprising at least methane, hydrogen and carbon oxides; and
h) introducing the partially reformed intermediate gas into the reactant side of the reaction zone of step c).
Preferred operating conditions are defined for feed gas and product gas temperatures and for the pressure differential across the membrane during operation of the process. For example, the reactant feed gas temperature is between 950xc2x0 F. (510+ C.) and 1400xc2x0 F. (760xc2x0 C.) and the product gas outlet temperature is greater than about 1500xc2x0 F. (815xc2x0 C.). The oxidant gas feed pressure is between 1 psig (0.069 barG) and 45 psig (3.1 barG). The reactant gas feed pressure is between 100 psig (6.9 barG) and 900 psig (62 barG) and the oxidant gas feed temperature is up to 200xc2x0 F. (111xc2x0 C.) greater than the reactant gas feed temperature. The oxidant gas feed temperature is preferably less than the oxygen-depleted oxidant gas temperature.
Applicants"" invention can be more readily understood by referring to the Detailed Description of the Invention and the Figures which are attached hereto.